The exhaust from internal combustion engines contains hydrocarbons, carbon monoxide, and nitrogen oxides which must be removed to levels established by government regulations. Most frequently, this is done by placing catalysts in the exhaust stream which are capable of removing all three contaminants at the same time.
Maximum conversion of all three exhaust gas contaminants can be achieved by a typical TWC catalyst only when the engine is operating in a narrow range of air/fuel ratios on either side of the stoichiometric value (typically a weight ratio of about 14.5/1). This optimum range is often referred to as the "A/F window." The engine is operated within the A/F window using an oxygen sensor as a reference to control the proper air/fuel ratio. Since the air/fuel ratio is controlled through a feedback control loop, there is a normal oscillation of the composition of the exhaust gases from rich to lean, i.e. the gases contain less or more oxygen than the target value. On the lean side, an excess of oxidants produces an oxidizing mixture and on the rich side an excess of reducing compounds is present. Exposure of the catalyst to net oxidizing or net reducing conditions can have a large effect on the performance of the catalyst, especially when ceria is present. Exposure to reducing conditions can lead to large activations of the catalyst. Net reducing conditions over extended periods are experienced by TWC catalysts during startup and during acceleration of vehicles using such catalysts.
Typical TWC catalysts contain small amounts of Group VIII metals, particularly platinum, palladium, and rhodium, supported on a high surface area support such as alumina. While gamma alumina is often preferred, other forms of alumina such as delta, eta, theta, and alpha alumina may be present. Other metal oxides such as zirconia, titania, and rare earth oxides have been suggested in the art as supports. For optimum performance the TWC catalysts will often contain promoters, such as the alkaline earth oxides including barium and strontium, transition metals including iron and nickel, and rare earth oxides including cerium and lanthanum. The function of these promoters is not fully understood but it is believed that they have several functions, among which is the stabilization of the supports to prevent or limit phase changes and loss of surface area.
Ceria is a well-known component of TWC catalysts. It is often referred to as an "oxygen storage component" since it is considered to have the ability to give up oxygen when the catalyst is exposed to reducing conditions and to reoxidize when exposed to oxidizing conditions. As has already been noted, the catalyst is operated under conditions which inherently swing between oxidizing to reducing as the oxygen sensor and its control system keep the air/fuel ratio within the desired operating A/F window around the stoichiometric value. It has also been suggested that ceria may stabilize the support structure, promote the activity of the precious metals, or promote the water gas shift reaction. See for example, B. Harrison, A. F. Diwell and C. Hallet, Platinum Metals Rev., 1988, 32, (2), 73-83.
In their report on the deactivation of three-way catalysts, Furabiki and Yamada, SAE Technical Paper Series 881684, studied the increase in platinum and ceria crystallite size as operating temperature was increased. They concluded that smaller ceria crystallites provided greater oxygen adsorption and better conversion of CO, HC, and NO.sub.x. However, it appears that the smallest ceria crystallites measured were about 200 .ANG..
In U.S. Pat. No. 4,791,091 Bricker discloses an auto exhaust catalyst having lanthanum crystallites of less than 25 .ANG. size which are produced by a unique process combining hydrosols of alumina with solutions of lanthanum compounds.
In their U.S. Pat. No. 4,868,149 Bricker et al. disclose another catalyst in which lanthanum is present in crystallites below 25 .ANG., but the lanthanum was deposited by impregnating a washcoated monolith with lanthanum nitrate. It is of interest with respect to the present invention that the crystallite size of ceria in such catalysts was reported to be 90-100 .ANG. where ceria was deposited on the alumina washcoat by an impregnation technique.
One method of applying the Group VIII metals is to employ an impregnating solution including a hydroxy carboxylic acid in order to cause the dissolved Group VIII metal compound to penetrate into the support. One example is found in Japanese published application J57119838 assigned to Mitsui Mining & Smelting. They disclose the impregnation of a washcoated ceramic honeycomb with an aqueous solution of palladium chloride and cerium acetate including 330-150 gm/liter of a hydroxyl carboxylic acid.
It has now been found that when the ceria is disposed as small crystallites they appear to promote the reduction of the precious metal contained in the catalyst. Apparently, this results in greatly enhanced activity when the ceria is applied according to the invention, as will be seen in the description below.